Drive shafts for vehicles, ships, etc. are generally composed of a metallic solid rod or hollow pipe having connected to both ends thereof a metallic fitting or joint element. With the recent demand for automobiles of reduced weight, not only replacement of metallic materials of automobile body with FRP but weight reduction of structural elements have been attracting attention. In particular, because a drive shaft is a revolving part, replacement of the material therefor with FRP is expected to make a great contribution to total weight reduction and has aroused considerable attention. That is, a drive shaft made of FRP has a weight reduced to a half to a quarter of that of the conventional steel-made one and has been extending its use in various automobiles.
In pursuit of comfortableness of ships, use of FRP-made drive shafts has been expected to deviate a resonant frequency from the service range. This is because FRP can make its resonant frequency either high or low while maintaining the torsional strength based on the fact that FRP is superior to metals, e.g., steel and aluminum, in specific strength (strength/density) and specific rigidity (modulus of elasticity/density) and that flexural rigidity or torsional rigidity of FRP can arbitrarily be controlled by changing the angle of orientation of fibers.
FRP-made drive shafts are generally produced by integrally molding a pipe made of FRP around the joint area of a fitting by filament winding or tape winding or by connecting a separately prepared hollow FRP pipe with a fitting by any means, for example, with an adhesive. However, the joint strength attained by the known method of connection is insufficient for transmission of high torsional torque or undergoes reduction with time. It is also known to shape the connecting portion into a regular polygon, but such shaping requires much labor in working, resulting in low productivity.
Other various means for transmission of high torsional torque has been proposed. For example, it has been suggested to engage a fitting with a pipe made of FRP (hereinafter referred to as an FRP pipe) both having serrations on the connecting portion thereof, or to fit a fitting having serrations on the connecting portion thereof into an FRP pipe to bite the inner wall of the FRP pipe as disclosed in JP-A-U-53-9378, JP-A-U-54-97541, JP-A-55-159311, JP-A-54-132039, and JP-B-62-53373 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"; the term "JP-A" as used herein means an "unexamined published Japanese patent application"; and the term "JP-B" as used herein means an "examined published Japanese patent application").
However, the former means encounters with a difficulty in forming serrations on the inner wall of an FRP pipe upon its molding. If serrations are formed by mechanical processing after molding the FRP pipe, the reinforcing fiber is cut to reduce the strength at the connecting portion, resulting in a failure to transmit a high torsional torque.
The same problem also arises in the latter means. That is, the reinforcing fiber at the connecting portion is apt to be cut by the serrations of the fitting, resulting in a failure to transmit a high torsional torque.
In an attempt to achieve reliable connection, it has been proposed to cover the joint with a metallic outer ring for reinforcement, but such diminishes the effect of weight reduction as purposed.